(1) Field of the Invention
The invention relates to the general technical field of anti-torque rotors for rotorcraft, in particular for helicopters. More precisely, the invention relates to the technical field of ducted tail rotor anti-torque devices with blades angularly distributed using phase modulation, and for use in helicopters, in particular. Such anti-torque devices differ from non-ducted, conventional anti-torque rotors that do not constitute the subject matter of the present invention.
The invention relates in particular to the above-mentioned anti-torque devices, also known as “Fenestron®” devices, in which a flow-guiding stator with stationary airfoil profile vanes is located downstream from an anti-torque rotor in the airflow passage or “duct”.
(2) Description of Related Art
Such anti-torque devices are known in particular from FR 2 719 549, which describes a relationship for angularly distributing the blades of the rotor so as to distribute sound energy as effectively as possible over the entire frequency spectrum, while taking account of the mechanical limitations on connecting the blades. The figures in that document show blades having shapes that are substantially rectangular in plan view. In addition, in order to avoid interaction between the wake of any one of the blades of the rotor with any one of the guide vanes taking place simultaneously over the entire span of the vane, the guide vanes are arranged in a non-radial manner. By way of example, each of the vanes is inclined relative to the radial direction by an angle that preferably lies in the range 1° to 25°, going from the axis of the passage towards its periphery and sloping in the direction opposite to the direction of rotation of the rotor. Such a configuration is favorable to taking up the torque which is exerted as reaction to the rotation of the rotor, on a central body located substantially coaxially in the passage and containing members for driving the rotor and members for controlling the collective pitch of the blades. Such torque takeup can then be withstood by the vanes situated in the passage.
A ducted anti-torque device provided with curved blades is also known from document WO 2006/110156. That document discloses curved blades combined with guide vanes that are offset radially and transversely in order to reduce the interaction noise between said blades and said vanes. Each of the described blades presents a geometrical shape with a single curve. An anti-torque device including such blades and such an arrangement of vanes is not capable of significantly improving its performance in order to approach the performance of a conventional anti-torque rotor.
Document U.S. 2004/0022635 or WO 2004/011330 describes a configuration of a concave/convex propeller blade following the model of a natural wave. The leading edge of such a blade presents a concave segment followed by a convex segment. The leading edge and the trailing edge are rounded so as to encourage the formation of an appropriate fluid flow around the surfaces concerned and so as to limit the formation of vortices in order to limit aerodynamic drag. As described, the best results are obtained by modeling the airfoil surfaces from a function that is sinusoidal or tangential to the curve representing the leading edge of each blade, with this being approximate since the amplitude is not defined. A blade presenting such a geometrical shape nevertheless appears to be unsuitable for improving the performance of a ducted anti-torque device. In other words, the definition of each blade does not present the required characteristics for satisfying the strict industrial requirements needed for making a ducted tail rotor anti-torque device for a rotorcraft.
Furthermore, the figures of said document U.S. 2004/0022635 show blades having an end chord that is smaller than the maximum chord of said blade.
Document EP 0 332 492 describes a blade for a high performance ducted propeller, in which the aerodynamically active portion is of rectangular shape. Its maximum camber increases from substantially 0 to substantially 0.04. Its relative maximum thickness decreases from substantially 13.5% to substantially 9.5%.
Document EP 2 085 310 describes a ducted anti-torque rotor in which the noise emitted at a frequency FE and perceived at a frequency FP is minimized. In that rotor, an airflow guide stator is located behind the blade path of said rotor (13).
Document WO 2009/54815 describes axial fan blades having airfoil surfaces with corrugations that give rise to suction and to pressure. The corrugations also increase the stiffness of the blades. In the figures, the blades are flared going from their roots to their free ends.
Document GB 212018 describes propellers made from a single piece of metal.